A couple of caveats - the WiiM Pro beta DLNA casting is coming up for 6 months old, and when I try it with my Ultra it doesn't see my Denon Ceol Piccolo, but does pick up my Linn amp and my other WiiMs. BubbleUPnP on Android and mConnect on ios see all my devices on the other hand.
You could add a WiiM Pro to each of your Denons then group them in the Google Home app and cast to that. Note that WiiM Minis couldn't be used as they don't support Chromecast even in a group where the main WiiM device does,